This 5-year mathematics professional development project involves 27 elementary teachers prepared and supported as Elementary Mathematics Specialists (EMSs) in high-need, urban schools. The EMSs are a distinctive population as informal teacher leaders, with a primary responsibility of teaching students. Described here are data collected at the end of Year 1 via a survey of coaching practices, a teacher leader record, and individual and focus group interviews. The findings illuminate the variety of ways they were serving as a more knowledgeable other and practicing agency in this teacher leadership. They were agentic in their teacher leader efforts by navigating constraints through: focusing on incremental changes; developing collegial, trusting relationships with peers; and leaning into the network of teacher support in the project. The findings also provide insights into how their primary and concurrent role as teacher of students provided credibility and understanding with fellow teachers, contributing to affordances in their informal teacher leader capacity.